In many applications, and partiCularly in the case of internal combustion engines for vehicles, cooling requirements are subject to wide variations depending upon operating conditions. When the engine is cold or cool, little or no cooling is required until the engine reaches a predetermined temperature. During engine operation, the degree of cooling required varies as a function of engine load and with external conditions such as air temperature and wind velocity. A cooling fan absorbs a considerable proportion of the total output power of an engine. From an engine design viewpoint, therefore, it is preferable to operate the cooling fan only when required or at a reduced level.
Various fan drives have been proposed to answer Industry needs. The basic Industry requirements being that the fan drive must be a modular unit which lasts substantially for the life of the engine and is easily serviceable. Early fan drive designs included sealed bearings and dry clutches. The contamination problems inherit with dry clutches are well known. Moreover, and because of their limited lubricant, sealed bearings would not and could not last the life of the engine. One problem with dry clutches is that they could not slip for long without overheating. Accordingly, they were limited to "on"/"off" applications. Furthermore, there was usually a shock load incurred in the drive unit when the dry clutch was engaged thus affecting the life expectancy of the clutch, attached belts, fans and idlers.
Having an appreciation for the drawbacks inherent with dry clutch fan drives, another attempted solution involved the use of a viscous coupling between the input and output members of the drive unit. It too had inherent drawbacks. First, viscous couplings have poor release capability and no lock-up capability. That is, a viscous coupling will not permit the drive input and output members to be driven at the same speed. Moreover, fan drives using viscous couplings were observed to have low horsepower capability and could not quickly dissipate heat build up in the unit. Most viscous coupling designs are slow to engage after sensing heat. Therefore, fan drives utilizing viscous couplings must initiate engagement at a lower temperature than desirable.
From these earlier designs, the invention disclosed in U.S. Pat. No. 3,804,219 evolved. This patented design uses shimmed bearings which are lubricated through a force feed system and a wet clutch assembly. This design outlasted the dry clutch systems, released and locked-up better than those drives using a viscous coupling and, by test, better stabilized the coolant temperature than any other fan drive. The '219 patented design, however, was not without drawbacks. This unique design employed fluids and gases for operation. Contaminants from both external and internal sources, however, made it difficult to keep the fluids and gases in the areas where they belonged. Moreover, all of the thrust load used in applying the clutch was also applied to the bearings of the drive assembly. As a result, bearing life was reduced. The bearings had to be shimmed to proper running clearance. The problems inherent with shimming bearings are well known. Moreover, the shaft supporting the drive unit was weakened because of the lubrication passages required for the bearings. Furthermore, the '219 invention utilized a spring applied clutch, the effectiveness of which was reduced by fluid pressure introduced into a controlled chamber. In addition to shimming the bearings, the spring had to be shimmed to proper position for stroke. This spring applied feature seldom, if ever, provided additional service time after a component failure. Moreover, the additional loading required to keep the spring compressed 95% of the time actually reduced the life of some other drive components. Furthermore, contaminants and cords from fraying belts continued to cause leakage by damaging the external seals. For these and other reasons, and as evidenced by the state-of-the-art, Industry continues to search for better fan drives.